Researchers in Scotland analyzed thousands of genes to see which ones make a protein that
prevents the cold sore virus from becoming active in 90 percent of people. Having identified
several genes, the researchers then looked for them in the blood samples of 20 people with cold
sores.

They found that the gene IL28B was consistently mutated in people who got sores.

The mutated gene is unable to produce the necessary protein, leaving the body’s immune system
unable to fend off the sores, said the researchers, writing in the journal PLoS Pathogens.